In a recent article in this journal,' S. J. Bai reported both wide-angle and low-angle x-ray diffraction from bundles of high performance polymer fibers. These included fibers of poly(p-phenylene terephthalamide) (PPTA), in particular the commercial materials Kevlar 29, Kevlar 49, and Kevlar 149 (Du Pont). The geometry of diffraction was unusual, being flat-film x-ray camera exposures taken in transmission with the x-ray beam parallel to the fiber axis. This was called axial x-ray scattering. The conclusions of the wide-angle scattering were also highly unusual; it was claimed that some fraction of the molecules in all these materials is oriented transverse to the fiber. This is despite the fact that conventional x-ray diffraction shows essentially no material in this orientation: and that the mechanical properties of these fibers are explained by a very high degree of axial alignment of the stiff molecule^.^*^
The explanation is that the interpretation presented in the article' is not correct; the results presented do not support the conclusion. The "axial x-ray scattering" method is not useful for samples that have fiber symmetry. Simple geometry shows that an x-ray diffraction pattern obtained in this way cannot give any more information than is already contained in the conventional flat-film fiber pattern (one taken in transmission with the fiber axis perpendicular to the x-ray beam). In the particular case of wide-angle scattering from PPTA fiber samples, the results of "axial x-ray scattering" can easily be interpreted to be in full agreement with the conventional alignment of molecules, closely parallel to the fiber axis.
Conventional diffractometry (with the fiber axis parallel to the diffractometer axis) and conventional flat-film fiber patterns are both much better ways of determining what crystal planes lie in the equatorial plane of the fiber. Determining the full orientation distribution of molecules requires several diffractometer scans or exposures with the fiber a t different orientations.
The fundamental problem with the article is that Bai assumed that the x-ray beam directed along the fiber axis gives information about lattice planes with their normals perpendicular to the fiber axis.
"Thus, lattice reflections from the axial scattering are expected to be identical to those along the equatorial direction from the conventional measurement. . ." (ref. 1).